A recovery boiler is a furnace wherein a waste fuel and air are combusted and chemicals from the waste fuel is recovered. In the pulp and paper industry one such waste fuel is called black liquor, which comprises in part water and sodium sulfate (Na.sub.2 SO.sub.4). The combustion of black liquor in a recovery boiler results, among other things, in a chemical process in which sodium sulfide (Na.sub.2 S) is recovered through the chemical reaction of the combustion process. In the pulp and paper industry, the recovery of sodium sulfide is essential to the paper manufacturer, inasmuch as the chemical is used in the chemical reaction to break the lignin of the fibers to produce pulp. For the pulp and paper industry then, the recovery boiler serves two functions, viz. an essential chemical of the paper producing process is produced from the recovery boiler, and a certain amount of energy is liberated for use to generate steam and/or electricity for use at the mill.
A recovery boiler comprises a fuel input, and a plurality of air inputs, a smelt output and an exhaust output. The air input, which is closest to the bed of the boiler in which air enters into the recovery boiler, is termed the primary air input. In the sequence of order of location of other air inputs into the boiler, the other air inputs into the boiler which are, in successive further distance away from the bed, termed secondary air inputs and tertiary air inputs, respectively. Fuel and air are primarily combusted in a zone which is located near the level of the secondary air input, and referred to as the oxidation zone. It has long been recognized that the primary air input is responsible for controlling the amount of air entering into the area just above the bed of the boiler, hence, for creating either a reducing atmosphere or an oxidizing atmosphere in the area just above the bed of the boiler (a reducing atmosphere being defined as an oxygen starved atmosphere; whereas, an oxidizing atmosphere is defined as an oxygen enriched atmosphere). The combustion of black liquor in a recovery boiler in a reducing atmosphere results in the following main chemical reaction: EQU 2C+Na.sub.2 SO4.about.2CO2+Na.sub.2 S
The molten state of sodium sulfide (Na.sub.2 S) which is recovered from the bed of the boiler is termed smelt. It has been recognized that for this chemical reaction to take place, a reducing atmosphere should be maintained in the area just above the bed, hereafter referred to as the reduction zone. If there is too much primary air above the bed, then the reduction efficiency is decreased since an oxidation reaction instead of a reduction reaction will take place. Moreover, the heat released by the oxidation (combustion process) will primarily be used to raise the temperature of the excess amount of primary air. The raising of the temperature of the excess amount of primary air will cause a large upward draft of air. The upward draft will cause the liquor droplets to be retained longer before hitting the bed. The longer the liquor droplets remain in its flight, the more water in the liquor will evaporate and the combustion process will have to proceed further prior to the liquor droplets hitting the smelt bed. These effects will result in a gradually cooling surface temperature of the bed leading toward an eventual extinction of the fire. On the other hand, if too little primary air is supplied, the combustion process will not proceed causing the temperature in the smelt to decrease making it difficult to drain. The bed will then start building up, increasing the rate of cooling and rapidly extinguishing the fire. Hence, a very critical measure of the performance of this zone is the temperature above the bed.
Heretofore, one method of measuring the temperature above the bed is to take a direct measurement of the temperature of the bed through an optical pyrometer. While this direct approach is in theory the best, practical implementation of this approach has led to many difficulties due in part to (1) the temperature of the bed which is at an extremely high temperature, typically on the order of one thousand degrees centigrade (1000.degree. C.) necessitating cooling means for the pyrometer; and (2) the dirty environment in which the pyrometer must operate, and thus, it is subject to reliability problems.
The main objective in recovery boilers is to dispose of a process waste material black liquor by burning the organic residue, thereby generating steam, and converting the inorganic chemicals to a reusable form. This is to be done while at the same time minimizing the carry-over of particulate matter and release of environmentally objectionable gases through the boiler's stack. There have been many attempts in the past to improve boiler efficiency by implementing complex control systems that affect airflow rate into the combustion chamber. Notable examples of this are shown in U.S. Pat. No. 4,362,269 issued to Rastogi on Dec. 7, 1982, and U.S. Pat. No. 4,359,950 issued to Leffler on Nov. 23, 1982. Both patents provide a good description of recovery boiler operation and recognize that boiler efficiency is affected by the control of air into the combustion chamber.
Most modern day recovery boilers have three levels where air, usually called "combustion air," is input to the boiler's furnace or combustion chamber. The lowest or primary level is at or near the same level as the burning bed. The mid or secondary level is positioned just above where the top of the burning bed would normally be located if the boiler were operating at optimum design capacity. The upper or tertiary level is located above the normal position where fuel guns deliver black liquor fuel into the combustion chamber. Some, but not all, older recovery boilers employ only two levels of combustion air, e.g., primary and secondary, with the secondary being high above the fuel guns.
Combustion air is delivered at the secondary and tertiary levels by windboxes which are essentially large, box-like ducts that are mounted to and surround the outside wall of the combustion chamber. A windbox is a large box having a plurality of openings or ports in a furnace wall leading into the boiler's combustion chamber. Pressurized airflow is provided to the windboxes by a fan, and each windbox consequently functions as a plenum. These ducts operate as manifolds and feed air directly into the combustion chamber through a number of ports in the chamber's walls.
In the past, combustion air exiting secondary or tertiary windboxes into the combustion chamber did not always have sufficient velocity or momentum to mix with upwardly exiting furnace gases. In fact, poor penetration of combustion air tends to channel high temperature gases into the center of the furnace (a stack pattern) resulting in inefficient combustion of materials, poorer liquor recovery, a high level of chemical carryover, higher TRS/CO emissions and higher furnace flue gas exit temperatures. All of these things are undesirable in a black liquor recovery boiler, as they reduce the unit's capacity.
As noted above the lowest air nozzles in the furnace wall are called primary air nozzles. They are positioned level with the surface of the char bed and therefore molten and unburned material from the bed may penetrate into the nozzles. Conditions on the level of the primary air nozzles are also otherwise highly corrosive, which shortens the service life of the nozzles. Furthermore, even great quantities of molten material may unexpectedly flow out of the char bed against the furnace walls, and the penetration of the molten material into the nozzles exerts a high strain on the nozzles. As a result, the nozzles are burned and corrode easily and have to be replaced subsequently.
Existing nozzles are typically made of a tube welded to the pressure casing of the recovery boiler. In some eases, the nozzle is surrounded by a refractory material to prevent damage by smelt leakages. The refractory material is provided either on the edges of the nozzle and below it, or it surrounds the nozzle. A problem therewith is that the nozzle can be replaced only by detaching the entire nozzle structure from the boiler wall. To achieve working conditions in which the detachment of the nozzles from the welds can be done, the shut-down of the boiler is necessary. Another problem is that the detachment of the nozzles may damage the boiler tubing, as a result of which operational disturbances and tube damages may occur after the replacement. If the nozzle is attached to the wall tubes of the furnace by welding, damage to the nozzle usually also results in damages to the furnace wall tubes to which the nozzle is attached.
In the secondary and tertiary nozzles, air flow may be disrupted by numerous factors including structural discontinuities in the windbox and nozzle design and exit port interface. This can lead to inefficient air flow, and in some instances, to reflux of the gas within the boiler into the nozzle and a portion of the duct work; thereby raising the temperature in the area externally of the boiler and significantly shortening the life of the plenum structure.